Metal-halide discharge lamp for photooptical purposes

ABSTRACT

A metal-halide discharge lamp for photooptical purposes has a small  electe spacing of less than 15 mm, preferably 2-8 mm, to provide an essentially pin-point light source, and a fill which contains AlI 3  in an amount between 0.1 and 4.5 mg/cm 3 . Other filling components may in particular be halides of mercury, indium, thallium or cesium; up to 2 mg/cm 3  of AlBr may be added. The lamp is particularly adapted for combination with a, preferably parabolic, reflector.

FIELD OF THE INVENTION

The invention is based on a metal-halide discharge lamp which can beused for instance for video projection, endoscopy, or medical practice(operating room lights), and which is especially suitable for videoprojection by the liquid crystal technique (LCD), and especially alsofor large television screens with an aspect ratio of 16 to 9. Typicalpower ratings are from 100 to 500 W.

BACKGROUND

The use of aluminum in the discharge vessel of lamps has already beenknown for a long time. However, it is problematic, in view of thehygroscopic performance of the aluminum compound in the filling processand the severe attack on the electrodes during the service life, whichgreatly limits the service life. Accordingly, the use of fillings thatcontain aluminum has until now been limited to either electrodelesslamps (U.S. Pat. Nos. 4,672,267 or 4,591,759, for example) or lamps inwhich the electrodes are especially coated in order to attain a suitablechemical reaction of the aluminum, see U.S. Pat. No. 3,914,636, to whichGerman Patent Disclosure DE-OS 24 22 576 corresponds.

A metal-halide lamp with wall loading of more than 40 W/cm² is known, inwhich a filling that contains either aluminum chloride or aluminumbromide is introduced into a discharge vessel that has activatedelectrodes, see German Patent 1,539,516. However, such fillings tend tomake for very short service lives, on the order of magnitude of 100hours. They are intended to generate a daylight-like spectrum, at thecost of high loading.

U.S. Pat. No. 5,220,237, Maseki et al., to which European PatentDisclosure EP-A 459 786 corresponds, describes a lamp for photoopticalpurposes with a long service life, particularly for video projection,which as filling components contains in addition to mercury and argoniodides of the rare earths dysprosium and neodymium and of cesium. Rareearth fillings were previously the only ones that were usual for suchlamps, because they assure good color rendition with a high light yield.This patent disclosure is hereby expressly incorporated by reference.

Although for general lighting rare earth fillings are quite suitable,they do not meet the high demands made of lighting for photoopticalpurposes. The reason for this is that large quantities of rare earthmetals attack the discharge vessel, which is typically of quartz glass,and at the high operating temperatures this gradually leads todevitrification and finally to the risk of bursting. The devitrificationworsens the optical characteristics of such lamps so considerably(diffuse projection of the arc) that the lamps can no longer be used forphotooptical purposes, where exact projection of the arc by the opticalsystem is critical. Finally, maintenance of these lamps is alsounsatisfactory. The light formation with rare earth metals also resultsprimarily from molecular electron transitions which thus occur at theedge of the arc, so that in the application for projection purposes, forinstance, color fringes can appear on the projection screen (poor coloruniformity).

THE INVENTION

It is an object of the present invention to create a lamp forphotooptical purposes that is distinguished especially by a long servicelife, good maintenance, and homogeneous color distribution, and whichhas good color rendition.

Briefly, in accordance with the invention, metal-halide lamps forphoto-optical purposes provide a color temperature of 5000 K and havethe combination of these features: an electrode spacing of 15 mm atmost; to create the most pinpoint possible light source, preferredvalues are between 2 and 8 mm. The color temperature is above 5000 K,and in particular is from 6000 to 10,000 K; and

The lamp was a filling that, as its essential or sole metal-halidecomponent, contains from 0.1 to 4.5 mg/cm³ of AlI₃. Adding aluminum inthis form to the lamp with the aforementioned small electrode spacinghas two advantages. First, accurate metering of even small quantities ofaluminum is possible, since the atomic weight of the partner in thecompound, iodine, is very high. Second, iodine specifically isespecially well-suited for the halogen cycle in this particular case,and it does not attack the electrodes as severely as chlorine orbromine. Another advantage is that this filling system is sononvulnerable that the same filling can be used for various wattagestages, without changing the color temperature. Finally, the influenceof the iodine on the lamp spectrum (absorption in blue) is desired.

Depending on the electrode configuration, it may also be advantageous toadd up to 2.0 mg/cm³ of AlBr₃.

Until now, AlI₃ was not considered to be very suitable, because thelight yield obtainable with it is relatively low (approximately 70lm/W), compared with conventional rare earth fillings (approximately 100lm/W). However, this failed to take into account the fact that the lightyield, referred to the total optical structure, or in other wordsmeasured in an associated reflector and with the greatest possibleparallelism of the light beam (angle of divergence<5°), becomessubstantially better compared with conventional systems, and thus theoverall system yield is comparable. This is because light formationtakes place by means of atom transitions, which occur predominantly inthe short arc core, thus considerably limiting the color separation.

An especially important advantage is finally that the color renditionattainable with AlI₃ is an especially good match for the profiledemanded. For video projection, what is known as the R/G/B distributionis an especially important parameter for determining color rendition.This is understood to mean the relative distribution of intensity inthree selected wavelength ranges, namely red (R), green (G) and blue(B). These ranges will be defined herein as follows:

R=600 nm to 650 nm

G=500 nm to 540 nm

B=400 nm to 500 nm.

Conventional fillings have an excessively high proportion in the greenrange (and to a lesser extent of the blue range), at the expense of theproportion of red; for instance, R/G/B=18:67:15.

With aluminum iodide as the basic component, because of the uniformityof its spectrum, R/G/B values can be attained that have a markedlyhigher proportion of red:

R=25% to 35%.

G=50% to 65%

B=8% to 18%.

As further filling additives for fine tuning, InI (or some other halideof indium) and possibly a halide of mercury (such as HgI₂, HgBr₂) in atotal amount of up to 2.0 mg/cm³, and preferably up to 1.0 mg/cm³, areespecially suitable. By means of halides of indium, the proportion ofblue can be finely tuned, for instance. Other suitable filling additives(up to 1.0 mg/cm³) are halides of thalium and/or cesium, for fine tuningof the proportion of green and for arc stabilization. Finally, a slightaddition of rare earth metals, preferably in metallic form, for fillingup the spectrum especially between about 500 and 600 nm is possible, inan amount up to 0.5 mg/cm³. Thulium and dysprosium, especially in anamount up to 0.1 mg/cm³, are preferred. This amount is so slight thatthe resultant devitrification is insignificant.

Preferred halides are in general iodine and/or bromine; a mixture thatis adapted in terms of geometry and volume inhibits electrodeconsumption.

One special advantage is that the electrodes in the present fillingrequire no special treatment whatever; that is, no coating (for instancewith scandium oxide or thorium oxide as known in the art) is necessary.Electrodes in which a coil is slipped onto a shaft, where the shaftmaterial is of tungsten doped with a material of lower electron affinity(such as ThO₂), while the coil is advantageously of updoped tungsten,are especially suitable.

For the bulb, quartz glass is suitable, especially a bulb pinched atboth ends, which is covered on one or both ends for instance with a heatcoating (such as ZrO₂). Under some circumstances, the homogeneity of thelight and color distribution can be improved, as known per se, by beingmade matte.

In principle, a bulb of ceramic material (Al₂ O₃), as already known forother lamp types, is also suitable. Advantageously, the lamp is puttogether with a reflector to make a structural unit, as described inU.S. Pat. No. 5,220,237 (European Patent Disclosure EP-A 459 786). Thelamp is then mounted approximately axially in the reflector. Thereflector is coated dichroitically, for instance.

The lamp is especially well-suited to projection technology based onliquid crystals, which is also suitable as the basis for high-definitiontelevision (HDTV). This technology requires lighting medium in the formof a discharge lamp with special properties, especially in terms of theoptimal balance of the R/G/B proportions, the usable light flux of thescreen, and the light density. Other characteristics are service liveslonger than 2000 hours, high maintenance (above 50%, as much aspossible) with respect to the color location and intensity, and the mostparallel possible light emission. High light density and maintenance ofthe color location and of the intensity is necessary because the opticalsystem efficiency in the final analysis is on the order of only 1 to 2%.Since the angular acceptance of liquid crystals (LCDs) is at a maximumof only 5°, extremely parallel light is necessary, which is the samething as saying that the demand is for the most pinpoint possible lightsource. In general, however, this shortens the service life of the lamp.Other substantial demands are for homogeneity of the color temperatureand of the distribution of lighting intensity on the projection screen.

A filling system having up to 4.5 mg/cm³ of AlI₃ and up to 2.0 mg/cm³ ofInI is especially suitable. Both components produce light by atomtransitions, so that color fringes are avoided here as well. One generaladvantage of the filling is that the color proportions and their ratiosvary only slightly over the service life.

In an especially preferred version, the lamp comprises a dischargevessel of quartz glass, pinched on both ends, with axially arrangedtungsten electrodes. This discharge vessel is installed in a paraboloidreflector with dichroitic coating; the diameter of the reflector isadapted to the diagonal of the liquid crystal array (LCD). The coatingof the reflector is equivalent to an optical band pass that reflects thevisible spectrum and transmits IR and UV components. Increaseduniformity in the distribution of color and intensity in the LCD planecan be attained by suitable matting of the discharge vessel. Often, aheat buildup coating is applied to one or both the vessel endssurrounding the electrodes. The lamp is operated with an electronicballast device, known per se, which also assures reignition while hot.

DRAWINGS:

Several exemplary embodiments will be described in further detail belowin conjunction with the drawings. Shown are:

FIG. 1, a schematic illustration of the lamp with a reflector;

FIG. 2, the spectrum of a lamp;

FIGS. 3-8, measurement findings with respect to the light flux, thecolor temperature, and the color location for various fillings.

FIG. 1 shows a metal-halide lamp 1 with a power of 170 W and a dischargevessel 2 of quartz glass, which is pinched on both ends at 3a, 3b,hereinafter, collectively 3. The discharge volume is 0.7 cm³. Theelectrodes 4, axially opposite one another, are spaced apart by adistance of 5 mm. They comprise an electrode shaft 5 of thoriatedtungsten, over which a coil 6 of tungsten is slipped. The shaft 5 isconnected, in the region of the pinched end 3, to an external power lead8 via a foil 7.

The lamp 1 is located approximately axially in a parabolic reflector 9,and the arc that develops in operation between the two electrodes 4 islocated at the focal point of the paraboloid. Part of the first pinchedend 3a is located directly in a central bore of the reflector, where itis retained in a base 10 by means of cement; the first power lead 8a isconnected to a screw-type base contact 10a.

The second pinched end 3b is oriented toward the reflector opening 11.The second power lead 8b is connected in the region of the opening 11 toa cable 12, which is returned in insulated fashion through the wall ofthe reflector back to a separate contact 10b. The power leads 8b arehereinafter collectively referred to as "8". The outer surfaces of theends 13 of the discharge vessel are coated with ZrO₂, for heat builduppurposes. The central portion 14 of the discharge vessel is matted, toimprove uniformity.

In a first exemplary embodiment, the filling of the discharge volume,besides 200 mbar of argon and mercury, contains the following:

1.15 mg of AlI₃

0.1 mg of InI

0.36 mg of HgBr₂.

The spectrum of this lamp is shown in FIG. 2. With it an R/G/B ratio of26:58:16 is attained. The wall loading is approximately 35 W/cm². In theprocess of filling the lamp with AlI₃, care should be taken to assurethe best possible purity, and especially to assure the absence ofoxygen.

In a second exemplary embodiment, 1.15 mg of AlI₃ is used, and in athird exemplary embodiment 1.15 mg of AlI₃ and 0.05 mg of Tm are used.The R/G/B ratio is then 29:55:16 and 28:57.5:14.5, respectively.

In a fourth exemplary embodiment, 0.05 mg of Tm are added to the firstexemplary embodiment. The R/G/B ratio attained is 26.5:57.5:16. Theresultant spectrum is shown in FIG. 8. There the spectrum without Tm(curve a) of FIG. 2 is compared with the Tm-containing filling (curveb). The thulium primarily causes a filling up of the spectrum between510 and 630 nm.

With these fillings, good color uniformity in the projection isattained, as well as excellent constancy of the color temperature T_(n)over a service life of 2000 hours; the maintenance is 70%. The colorlocation is x=0.295 and y=0.317.

The color temperature T_(n) can be adjusted by varying the quantity ofAlI₃, with starting values of T_(n) of between 6000 and 10,000 K.

Particularly good results in terms of service life and maintenance canbe attained with the following fillings:

0.45-3.3 mg/cm³ of AlI₃

0-0.3 mg/cm³ of In halide, especially InI

0-0.7 mg/cm³ of Hg halide, especially HgBr₂

0-0.7 mg/cm³ of halides of Cs and/or Tl

FIGS. 3 and 4 show the maintenance of the light flux within an angle of5° (so-called panel lumens) in relative units, and the course of thecolor temperature, in each case over a lamp burning time of more than2000 h, for various fillings in a 170 W lamp (volume, 0.7 cm³). Thedischarge vessel was coated with ZrO₂, but without matting. The variousfillings are:

A) 2.3 mg of AlI₃, 0.1 mg of InI, 0.36 mg of HgBr₂

B) 1.15 mg of AlI₃, 0.1 mg of InI, 0.36 mg of HgBr₂

C) 0.6 mg of AlI₃, 0.1 mg of InI, 0.36 mg of HgBr₂

D) 0.3 mg of AlI₃, 0.1 mg of InI, 0.36 mg of HgBr₂

It can be seen from FIG. 3 that the maintenance after 2000 hours is onthe order of magnitude of 60 to 75%. After 3000 hours, it is still 50 to65% and thus still meets the minimum requirements. The absolute value ofthe light flux is the highest with a low dose of Al (D), and itdecreases as the dose of Al rises. The dropoff over the course of theburning time is approximately independent of the quantity of aluminum.

In FIG. 4, the color temperature T_(n) is inversely proportional to thedose of aluminum. It is extremely constant over the burning time. Ingeneral, color temperatures of around 8000 K are preferred for videoprojection, corresponding to a dose of 0.6 to 1.15 mg, which isequivalent to a volume-independent dose of 0.85 to 1.65 mg/cm³.

If both these drawings are studied together a major advantage of thesefillings become clear, namely the different demands, for instance withrespect to the color temperature, can be met without major changes inthe filling, except for the quantity of AlI₃, or in other technicalproperties of the lamp.

FIG. 5 for filling B) shows the color location (x or y value) as afunction of the service life (starting value after 1 hour, value after1000 and 2700 h) and of the location (nine measuring points E1-E9, whichare located uniformly over the area of the projection screen in a 3×3matrix). The x value fluctuates only slightly between x=0.28 and x=0.29,while the y value fluctuates between y=0.295 and 0.31.

In FIGS. 6 and 7, finally, the performance of a 200 W lamp is shown,which is otherwise similar in design to the 170 W lamp. The fillingsused here are in one case identical to filling C); in the other, thefollowing filling E) was used:

E) 0.9 mg of AlI₃, 0.1 mg of InI, 0.36 mg of HgBr₂.

FIG. 6 shows the lighting intensity on a projection screen in lux,averaged over the grid of nine measuring points described in FIG. 5 as afunction of the burning time, while FIG. 7 shows the color temperatureas a function of the burning time.

Once again, the nonvulnerability of the filling system based on AlI₃with respect to special adaptations to special demands is confirmed.

In general, the addition of slight quantities of rare earth metals canshorten the service life of the lamps of the invention somewhat. This iscompensated for, however, by an increase in the light yield (by up to10%) and a lowering of the color temperature (by as much as 500 K).

We claim:
 1. A metal-halide discharge lamp for photooptical purposeshavinga translucent discharge vessel (2); two spaced electrodes (4)which face one another within the vessel and which are connected topower leads (8) extending to the outside, characterized by anarrangement for providing a light source which is close to a pin-pointlight source and which provides light at a color temperature of at least5000 K, wherein said arrangement comprises an electrode spacing of amaximum of 15 mm; and a filling within the vessel which comprises 0.1 to4.5 mg/cm³ of Al I₃, as the essential or sole metal-halide component forlight generation by said light source; and 0 to 2.0 mg/cm³ of halides(Ha) of indium (InHa) and mercury (HgHa₂), or halides (Ha₂) of mercury(Hg).
 2. The lamp of claim 1, characterized in that the fillingadditionally contains up to 1.0 mg/cm³ of halides of thallium (TlHa) andcesium (CsHa₂), or halides of cesium (CsHa).
 3. The lamp of claim 2,characterized in that the electrode spacing is between 2 and 8 mm. 4.The lamp of claim 2, characterized in that the lamp forms a structuralunit with an optical reflector (9), optionally an essentially parabolicreflector; andthat the electrode spacing is between 2 and 8 mm.
 5. Thelamp of claim 1, characterized in that the lamp forms a structural unitwith an optical reflector (9), optionally an essentially parabolicreflector.
 6. The lamp of claim 1, characterized in that the electrodes(4) are made from tungsten, and the electrode or a portion thereofoptionally is doped with a material of low electron affinity.
 7. Thelamp of claim 6, characterized in that the electrodes (4) are uncoated.8. The lamp of claim 6, characterized in that the lamp forms astructural unit with an optical reflector (9), optionally an essentiallyparabolic reflector; andthat the electrode spacing is between 2 and 8mm.
 9. The lamp of claim 1, characterized in that the electrode spacingis between 2 and 8 mm.
 10. The lamp of claim 1, characterized in thatthe discharge vessel (2) is a quartz glass bulb pinched at both ends,which is optionally coated in its entirety or partially.
 11. The lamp ofclaim 1, characterized in that the lamp forms a structural unit with anoptical reflector (9), optionally an essentially parabolic reflector;andthat the electrode spacing is between 2 and 8 mm.
 12. The lamp ofclaim 1, characterized in that the filling additionally contains up to2.0 mg/cm³ of AlBr.
 13. The lamp of claim 12, characterized in that theelectrode spacing is between 2 and 8 mm.
 14. The lamp of claim 12,characterized in that the lamp forms a, structural unit with an opticalreflector (9), optionally an essentially parabolic reflector; andthatthe electrode spacing is between 2 and 8 mm.
 15. The lamp of claim 1,characterized in that the filling additionally contains up to 0.5 mg/cm³of rare earth metals.
 16. The lamp of claim 15, characterized in thatthe electrode spacing is between 2 and 8 mm.
 17. The lamp of claim 15,characterized in that the lamp forms a structural unit with an opticalreflector (9), optionally an essentially parabolic reflector; andthatthe electrode spacing is between 2 and 8 mm.
 18. The lamp of claim 1,characterized in that over three selected wavelength ranges R/G/B,whereR=600 nm to 650 nm G=500 nm to 540 nm B=400 nm to 500 nm, therelative light intensity distribution amounts to R=25% to 35% G=50% to65% B=8% to 18%.
 19. The lamp of claim 18, characterized in that theelectrode spacing is between 2 and 8 mm.
 20. The lamp of claim 18,characterized in that the lamp forms a structural unit with an opticalreflector (9), optionally an essentially parabolic reflector; andthatthe electrode spacing is between 2 and 8 mm.